My periods are delayed, is it pregnancy?

Last month i take the period delaying medicine Like i took the medicine on 6th of that month at 12Pm then 8 pm and next morning 7am but still i received my periods 2-3 days later at the accurate date of periods(10th-15th of that month).But this month i do sex on 4th of this month And today is 18th of this month and i still dont get my periods. I ook the pregnancy kit urine test on 16th of this month (the day before yesterday) It shows Negative results that i'm not pregnant My periods sometimes get delayed and sometimes gets early.This time i'm very confused Am i pregnant? My periods everytime were like Actual date of periods was 28th But some times i got periods on 28th of month and next month they will come on till 20th Then next month till 10th That's why i'm so confused What should i do now to be sure that i will receive periods or not. Please tell me some symptoms or any thing that clear my doubt and give me accurate answer that am i pregnant or will i get my periods Please.